The FBI is following up on leads and tips concerning the practice of female genital mutilation in the US, where it says more than 500,000 women and girls are at risk of being subject to the controversial procedure.

Female genital mutilation, also known as “cutting” or FGM, involves the partial or total removal of a female’s external genitalia. The procedure has been traditionally performed in the Middle East and Africa as a cultural custom, though it does not offer any medical or health benefit.

Despite being made illegal by Congress back in 1996, roughly 513,000 women and girls in the US were at risk of undergoing FGM as of 2012, according to a recent study from the US Centers for Disease Control and Prevention. That is more than three times higher than previous estimates based on data from 1990.

Notably, the CDC report said that the number of women and girls at risk below the age of 18 had increased by more than four times, making up about one-third of potential victims.

“We believe some of it is being conducted by medical practitioners – physicians, nurses, midwives – and some by female elders within the communities who have the distinction of being what is called a cutter,” said Special Agent Kerry Sparks, who focuses on FGM cases as part of the FBI’s International Human Rights Unit (IHRU), in a statement.

The FBI now says it is currently looking into leads on the practice and hopes that community members will pass on information about potential cases. Women and girls most at risk generally live in major urban areas such as New York, Minneapolis, Los Angeles, and Washington, DC, the Bureau said.

According to the CDC study, the rise “was wholly a result of rapid growth in the number of immigrants from FGM/C-practicing countries living in the United States.”

The FBI said those practicing FGM generally argue that they are observing deeply rooted traditions and beliefs, but noted that the UN has called cutting “an extreme form of discrimination against women and girls.”

Although the practice was banned in the US in 1996, some women have been sent abroad to have the procedure performed, compelling Congress to pass additional legislation to make “vacation cutting” illegal as well.

In February, the FBI and US Customs and Immigration Enforcement both recognized the International Day of Zero Tolerance for Female Genital Mutilation by calling for the custom’s “total eradication” and warning that anyone suspected of being involved in it – including those sending girls overseas to be cut – could be prosecuted by the Human Rights Violators and War Crimes Center.

President Barack Obama also highlighted the issue during a visit to Kenya in July of 2015.

“There’s no excuse for sexual assault or domestic violence, there’s no reason that young girls should suffer genital mutilation, there’s no place in a civilized society for the early or forced marriage of children,” Obama said. “These traditions may go back centuries; they have no place in the 21st century.”